Examination technology which utilizes radiation can examine inside a subject in a non-destructive fashion. In particular, the radiological examination technology to a human body includes an X-ray CT, a PET, a single photon emission tomography apparatus (Single Photon Emission Computed Tomography, hereinafter referred to as “SPECT”) and the like.
Any of these techonologies is a technology of measuring the physical quantity of an object for examination as an integration value in the direction of flight of radiation, and bringing the integration value thereof into back projection, and thereby calculating and imaging the physical quantity of each voxel in the object to be examined. These technologies require processing of large data, and the rapid progress of computer technology in the recent years has been accompanying provision of rapid and highly detailed images.
A PET as well as an SPECT being a radiological imaging apparatus is a technique capable of detecting functions and metabolism on the level of molecular biology which cannot be detected with an X-ray CT and the like, and is capable of providing function images of a body. The PET is a technique of administering a radiopharmaceutical which has been labeled with a positron-emitting radionuclide such as 18F, 15O and 11C, of measuring its distribution and of implementing imaging. The pharmaceutical is fluoro-deoxy-glucose (2-[F-18]fluoro-2-deoxy-D-glucose, 18FDG), etc. and this utilizes that a pharmaceutical is highly concentrated at tumor tissues with saccharometabolism, and is used to identify a tumor site.
The radionuclide taken by a body decays to emit positron (β+). The emitted positron is coupled with electron and is annihilated, then emits a pair of annihilated γ rays (annihilated γ ray pair) respectively having 511 keV energy. Since these annihilated γ ray pair are emitted in the approximately opposite directions (180°±0.6°), a plurality radiation detectors disposed so as to surround the periphery of the object to be examined detect the annihilated γ ray pair, accumulate data on their emitted directions and thereby can derive projection data. Bringing the projection data into back projection (using the above described filtered back projection method and the like), identification and imaging of emitted position (concentration position of radionuclide) will become feasible.
SPECT is a technique of administrating a radiopharmaceutical which is labeled with single-photon-emitting radionuclide, of measuring distribution thereof and of imaging. From the radiopharmaceutical, a single γ ray with energy around 100 keV is radiated so that this single γ ray is measured with a radiation detector. Since measurement of a single γ ray cannot identify the direction of its flight, an SPECT is provided with a collimator which is inserted in the front plane of a radiation detector, and detects only the γ ray from a specific direction and thereby derives projection data. Likewise the PET, utilizing filtered back projection method and the like, the projection data are brought into back projection to derive image data. Different from the PET, the coincidence due to measurement of a single γ ray is not needed and a smaller number of radiation detectors will do, etc. and therefore the configuration of the apparatus is simple.
The above described radiological imaging apparatus such as conventional PET and SPECT, etc. uses a scintillator as a radiation detector in order to derive an image. A scintillator implements processing to temporarily convert the incident γ ray into a visible light and thereafter to convert further into an electric signal with a photomultiplier (photomul). A scintillator is not abundant in photon generation at the time of visible light conversion and needs a two-step conversion process as described above, therefore giving rise to a problem that it has a low energy resolution and cannot always implement highly accurate imaging. A decrease in energy resolution, in particular, results in inability of quantitative assessment at the time of 3-D imaging of the PET. The reason is that, due to a low energy resolution, the energy threshold of γ ray is obliged to be lowered, resulting in detection of a lot of in-body scattering being noises which increase at the 3-D imaging.
Therefore, in recent years, much attention is being paid to the use of semiconductor detector as a radiation detector for a radiological imaging apparatus. A semiconductor detector converts the incident γ ray directly into an electric signal, and is characterized in a high energy resolution due to abundance in generated electrons and hole pairs.
Normally, it is known that features such as time resolution and energy resolution of a scintillator and a semiconductor detector decrease under environments with a high temperature, and a radiological imaging apparatus comprising a cooling mechanism as means therefor is disclosed (see, for example, JP-A-10-160847 and JP-A-9-276262).
PET examinations detect an annihilated γ ray pair and therefore need to determine coincidentalness of a detected event (carry out coincidence). At detection time of an annihilated γ ray pair, fluctuations exist due to noises and the like in radiation detector and circuit systems, and therefore in order to determine coincidentalness, an allowable specific coincidence time window is provided to determine that the detected two events within this coincidence time window are coincidental.
On the other hand, for a radiological imaging apparatus, in order to improve image qualities and improve quantitativeness of image information, features of time resolution and energy resolution in the above described scintillator and semiconductor detector should be improved.
Improvement of the feature of time resolution will be able to shorten the above described coincidence time window. Then, probability of spontaneously catching a γ ray that is not a true annihilated γ ray pair will be reduced. Since the spontaneously caught γ ray pair (random coincidental events) does not hold true positional information, exclusion of such noise components will improve image qualities and quantitativeness of image information. In addition, improvement of the feature of energy resolution will be able to exclude γ ray due to in-body scattering as described above and improve image qualities and quantitativeness of image information.
However, accompanied by technical advantages offered by radiological imaging apparatuses, increase in number and high density of radiation detectors is in progress, in addition, under the circumstance that the densified state of internally incorporated electronic circuit equipment, etc. accompanied by miniaturization of apparatus is in progress, application of the above described conventional cooling mechanism is unable to sufficiently cool the heat generated from electronic circuit equipment (signal processing apparatus) inclusive of a radiation detector, consequently, there was a concern that features of time resolution and energy resolution decrease.